Kaposi's sarcoma (KS) is a multifocal tumor characterized by proliferating spindle cells of endothelial cell origin, erythrocyte-filled neovascular slits and an inflammatory cell infiltrate. KS is the most frequent cancer arising in HIV-infected individuals and is one of the most common neoplasms of children in developing countries. Kaposi's sarcoma-associated herpesvirus (KSHV or HHV-8) is the etiologic agent of KS, and drives tumor development via direct infection of lesional endothelial and spindle cells. Dermal microvascular endothelial cells (DMVEC) infected with KSHV in vitro harbor a primarily latent infection with a fraction of cells expressing lytic viral genes, a pattern that reflects viral gene expression patterns in vivo. KSHV-infected DMVEC develop a spindle morphology and a transformed phenotype and cellular gene expression profiling reveals viral induction of a number of potentially tumorigenic genes. One of these genes, the proto-oncogene c-Kit, was shown to play an essential role in DMVEC transformation in vitro and a preliminary analysis of KS tissue confirmed expression of c-Kit in KS tumors in vivo. The long-term goals of this project are to clarify the scope and temporal kinetics of c-Kit expression in KS tumors, and to clarify the viral and cellular mechanisms that lead to c-Kit expression and c-Kit-mediated cellular transformation. Specific Aim 1 will use immunohistochemical and RT-PCR analysis to evaluate c-Kit expression in KS tumor tissue at early (patch and plaque) through late (nodular) stages of development in conjunction with serologic detection of latent or lytic KSHV infection. Specific Aim 2 will use in vitro-infected DMVEC to identify the KSHV gene(s) responsible for c-Kit induction, and the molecular mechanisms involved. Specific Aim 3 will use in vitro-infected DMVEC to elucidate downstream signaling pathways activated by c-Kit that lead ultimately to KSHV-induced transformation. For Aims 2 and 3, adenovirus vectors will be used to overexpress viral and cellular genes of interest. Gene function will be validated with antisense oligonucleotides or pharmacologic inhibitors and appropriate functional assays, c-Kit has proven to be an effective therapeutic target for treatment of gastrointestinal stromal tumor (GIST). Clarification of the role of c-Kit in KSHV-associated endothelial transformation may suggest a similar approach for treatment of KS.